For the study and detection of neutrons in space environments such as planetary and earth orbiting missions semiconductor silicon diode detectors have been characterized and calibrated at various thermal neutron sources. Two types of diodes adapted for thermal neutron detection were investigated: silicon MESA planar detectors equipped with thin 6LiF layers and silicon heterodiodes with a layer of natural boron or enriched 10B. The response and absolute detection efficiency have been measured. The influence of bias voltage and converter layer thickness were studied. As neutron sources we used a homogenous isotropic thermal neutron field by a set of PuBe radionuclide sources placed in a graphite pile as well as a parallel thermal neutron beam with high Cd ratio (105) and suppressed gamma background. Depending on the converter layer thickness and/or boron layer thickness as well as the choice of the threshold level, efficiencies of approximately 1% are obtained for both the silicon diodes with thin 6LiF and the boron rich silicon detectors. These values guarantee optimal stability of operation in remote and different environments as well as maximum signal-to-noise ratio by enhanced suppression of unwanted signals and gamma background.